Device for managing pulses in pump-probe spectroscopy

Abstract

A device for managing light pulses for measuring the reaction of a sample exposed to a first light pulse, the measurement being performed by analysis of a signal emitted by the sample subjected to a second light pulse, shifted with respect to the first pulse by a determined interval of time, the device including two optical detectors for detecting the pulses of two light beams emitted by two pulsed laser sources, respectively, each beam emitting pulses with respective repetition frequencies that are different, arbitrary and stable over a determined period in the direction of the sample; the detectors being connected to a computer for determining the interval of time between two pulses coming from the first and the second beam, respectively, and constituting the first and second pulses; the computer being connected to an analyzer for measuring the reaction of the sample having as input parameter the interval of time between the two pulses, where the computer uses an algorithm making use of the stability of the repetition frequencies for determining the interval of time.

Claims

1. A device for managing light pulses for measuring a reaction of a sample exposed to a first light pulse called a pump pulse, the measurement being performed by analysis of a signal emitted by the sample subjected to a second light pulse, called a probe pulse, temporally shifted with respect to the pump pulse by a determined time interval, the device comprising: two ultrashort pulsed oscillators of different pulse frequencies, stable over a determined period t, said oscillators being characterized by their relative phase which allows to foresee the delay between all the pulses produced by the two oscillators, two optical detectors beamed to detecting the pulses of two light beams emitted by two ultrashort pulsed oscillators respectively, the first beam being called the pump beam and the second beam being called the probe beam, in the direction of said sample; the detectors being connected to a computer configured to accumulate during said determined period t measurements of the relative phase and determine, based on these measurements, the value of the time delays between subsequent pump and probe pulses, with a precision improving with the square root of the number of measurements, hence in the picosecond or sub-picosecond domain, an acquisition system connected to the computer and configured to receive from the computer the time delay values and to acquire signals emitted by the sample in order to analyze its reaction to the pairs of pulses for determined values of time delay.

2. The device as claimed in claim 1, comprising two pulse selectors positioned on each of the two beams, respectively, each selector being configured to transmit toward a sample a particular pulse of the beam on which it is positioned.

3. The device as claimed in claim 2, wherein the pulse selectors are light amplifiers.

4. The device as claimed in claim 3, wherein the amplifiers are configured to amplify the pairs of pump-probe pulses having the predetermined value.

5. A method for measuring a reaction of a sample exposed to a first light pulse called a pump pulse, the measurement being performed by analysis of a signal emitted by the sample subjected to a second light pulse, called a probe pulse, temporally shifted with respect to the pump pulse by a determined time interval, both light pulses being emitted by ultrashort pulsed oscillators having different repetition frequencies that are arbitrary and stable over a determined period t, the method comprising: repeated measurements of the relative phase of the two oscillators during the period t; the determination, based on these measurements, of the value of the time delays between subsequent pump and probe pulses, with a precision improving with the square root of the number of measurements, hence in the picosecond or sub-picosecond domain; and the analysis of the reaction of the sample with an acquisition system for pairs of a pump pulse and a subsequent probe pulse of determined values of time delay.

6. The method as claimed in claim 5, wherein the predetermined value of time interval is transmitted to the acquisition system.

7. The method as claimed in claim 5, comprising selecting at least one pair of pump and probe pulses having a determined value of time interval.

8. The method as claimed in claim 5, comprising controlling, during the selection selectors to let through and amplify only said at least one pair having the predetermined value of time interval.

9. The device as claimed in claim 1, wherein the optical detectors are fast photodiodes, the electrical signal from which is sent to an electronic card of the computer configured to detect coincidences between pairs of a pulse of the first beam and a pulse of the second beam with a temporal precision limited by the electronics, resulting in an accuracy in the order of divided by the square root of n, where n is the number of measurements performed during the stability period t of the two oscillators.

10. The method as claimed in claim 5, comprising the measurement of the relative phase even when there is no coincidence between a pulse of the first beam and a pulse of the second beam by means of optical detectors each having one photodiode configured to transform each pulse into an electrical signal acting as input signal for an electronic time-to-digital converter (TDC) circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood on reading the following description, made only by way of example, and with reference to the appended figures, in which:

(2) FIG. 1 is a schematic view of a pump-probe spectroscopy system having a device according to a first embodiment of the invention;

(3) FIG. 2 is a timing diagram for the pulses of two lasers having different repetition frequencies according to the principle of asynchronous scanning, and showing the coincidences;

(4) FIG. 3 is a schematic view of an electronic assembly for an acquisition card of the device in FIG. 1;

(5) FIG. 4A is a timing diagram for the electronics in FIG. 3 showing a coincidence signal in a phase where the two lasers are not in coincidence;

(6) FIG. 4B is a timing diagram for the electronics in FIG. 3 showing a coincidence signal in a phase where the two lasers are in coincidence;

(7) FIG. 5 is a flowchart for the operation of the device in FIG. 1; and

(8) FIG. 6 is a schematic view of a pump-probe spectroscopy system having a device according to a second embodiment of the invention.

DETAILED DESCRIPTION

(9) With reference to FIG. 1, a device operating on the principle of heterodyne sampling conventionally comprises a pump pulsed laser source 1 and a probe pulsed laser source 3 emitting a pump beam and a probe beam, respectively. The two laser sources are typically femtosecond lasers, thus emitting pulses in the order of one hundred or so femtoseconds. The pump and probe pulse durations are equal or unequal. Similarly, the central wavelengths of the pump and probe beams are equal or unequal depending on the measurement to be performed.

(10) Each beam passes through an optical amplifier 5, 7 before being combined by a combiner 9. For example, the combiner 9 comprises a mirror and a semi-transparent plate.

(11) The beams thus combined are then directed toward an experimental device 11 in which a sample for measurement is placed.

(12) The response of the sample is received by a photodetector 13 then transmitted to an acquisition system 15.

(13) In the embodiment of the device for managing light pulses in FIG. 1, a sample slide 17, 19 is installed on the path of each beam at the output of each laser source 1, 3 so as to pick up a trace of each pulse on photodetectors 21, 23. Those skilled in the art will understand that the transparency of the slides is chosen so as to send to the photodetectors 21, 23 only the power necessary to their operation.

(14) The photodetectors 21, 23 are connected to a computer 25.

(15) The computer 25 comprises a control interface for the optical amplifiers 5, 7 and is connected to the acquisition system 15.

(16) The operation of the device is as follows.

(17) In general, two oscillators designed for free operation produce two combs of pulses 31, 33 of different and highly stable frequencies. The relative delay, or relative time phase, between the two combs progressively travels between the moment when a coincidence occurs and the following coincidence according to the principle of asynchronous scanning, FIGS. 2.

(18) .sub.1 and .sub.2 being the respective frequencies of the lasers, it is possible to define a time phase for each of the lasers: .sub.1=.sub.1 (t.sub.1) and .sub.2=.sub.2(t.sub.2) (.sub.1 and .sub.2 defining the time shift of the two pulse trains and t being the time coordinate). These laws being known, it is then possible to foresee with precision the relative phase =.sub.2.sub.1, and therefore the delay between all the pulses produced by the two oscillators.

(19) To measure the phase laws of the two oscillators, the computer detects the coinciding pulses while precisely counting the number of pulses produced by each of the two lasers between the coincidences. This evolution law for the interval of time between two pulses is therefore determined by interpolation or extrapolation. This makes it possible to determine .sub.2/.sub.1, in particular, with very high precision.

(20) Indeed, let t be the interval of time during which it may be supposed that the oscillators remain perfectly stable, that is to say that there is no measurable jitter phenomenon. With current oscillators, t is typically in the order of 1 ms. T is the period of the pulse train and is equal to around 10 ns for 100 MHz oscillators, and is the temporal precision of the coincidence measurement.

(21) The number of pulses produced during the period t has a value of N=t/T, which therefore corresponds to a number of coincidences in the order of n=N/T. Taking into account the stability of the oscillators, the precision over the measurement of the parameters .sub.1 and .sub.2 is therefore in the order of /{square root over (n)}. The ultimate precision of the device is therefore linked to the method used for detecting the coincidences.

(22) In the embodiment in FIG. 3, the detection of the coincidences is totally electronic.

(23) The photodetectors 21, 23 are fast photodiodes, the electrical signal from which is sent to an electronic card of the computer 25.

(24) The electrical signals are sent to two flip-flops D 41, 43. One of the signals is used as a common clock, and the other signal is distributed successively to the input of the first flip-flop and then of the second flip-flop with a slight delay, for a first delay line 45 and for a second delay line 47. At each new clock pulse, a coincidence is signified by a state difference between the outputs of the flip-flops.

(25) FIGS. 4A and 4B show the timing diagram for the signals 51, 53 at the output of the delay lines 45, 47 and for the signal 55 at the output of the photodetector and acting as clock for the two flip-flops D 41, 43. The signal 57 shows the signal at the output of the combinational exclusive OR for the two outputs of the flip-flops D. FIG. 4A shows the signals when the two lasers are not in coincidence, and the signal 57 therefore remains at 0, and FIG. 4B shows the signals when a coincidence occurs, the signal 57 then flipping from 0 to 1.

(26) The inventors have produced a prototype according to this outline which allows a precision in the order of 30 ps. Two optical fibers coupled to two Si PIN photodiodes have been connected by differential pairs to the coincidence detector and to the computer embedded in an FPGA of the Spartan family from the manufacturer Xilinx. With the numerical values above, this gives a rate of 300 coincidences per millisecond. Laser frequencies that are sufficiently stable with respect to this rate allow these coincidences to be averaged to obtain a final precision of slightly less than 2 ps. This value can be further improved by using a specialized circuit (ASIC) or by increasing the coincidence rate by reducing the number of electronic paths with different and known delays.

(27) As a variant, it will be noted that it is also possible to use known devices of the time-to-digital converter type.

(28) This makes it possible to make use of pulse pairs even when there is no coincidence and therefore to increase the number n of acquisitions contributing to the measurement and thus to improve the precision of the latter.

(29) Thus, in FIG. 5, having taken, in step 51, a precise measurement of the phase laws of the two oscillators, the computer is then capable of determining, in step 53, the value of the time shift between any pulse of the pump signal and any subsequent pulse of the probe signal. And therefore, for a determined shift value, it is capable of selecting, in step 55, a pair of pump-probe pulses having this value of time shift.

(30) The computer 25 then controls, in step 57, the amplifiers 5, 7 to let through, and amplify only, the corresponding pulses. In parallel, in step 59, it informs the acquisition system 15 of the time between the pump pulse and the probe pulse.

(31) As indicated in the introduction, in pump-probe spectroscopy, the aim is to measure the reaction of a sample over a varied range of times separating the pump pulse from the probe pulse so as to study the dynamics of the sample.

(32) Owing to the device described, it is enough to preliminarily configure the range of times to be studied, for example from 1 ns to 1 ms, and the number of samples, for example with a step of 1 ps, and the computer is then capable of selecting the series of pulses covering the requested range. It will be noted that the measurements can be in any order and can depend only on the sequence of the pulses in such a way as to minimize the measurement time. Therefore, the computer 25 is linked to the acquisition system 15 so as to supply the latter with information about the time between pulses corresponding to the measurement in progress. Simple sorting processing over the duration of the times then makes it possible to order the measurements. A beneficial variant comprises not using the selectors 5 and 7, which makes it possible to make use of all the pulse pairs produced by the two oscillators 1 and 3, the delays being determined a posteriori by the computer. In this case, the temporal dynamics are limited to the period of the oscillators, typically in the order of 10 ns.

(33) In a second embodiment, FIG. 6, the photodetectors 21, 23 and the electronic acquisition card are replaced by an optical device.

(34) The latter comprises an interferometer 61, possibly a fiber interferometer, on which the two laser beams are aligned. The coincidences corresponding to the temporal superposition of two pulses are detected by the optical linear interference generated. The interference signal is detected with a photodiode 63 and sent to an acquisition card in the computer 25, which compares it with a threshold value. As a variant, it is possible to use a photodiode on each of the two outputs of the interferometer to carry out differential detection.

(35) Optical detection of coincidences as described makes it possible to have a precision in the order of the duration of the pulses, i.e. typically 100 fs. With the numerical values above, this gives a coincidence rate n/t in the order of one coincidence per millisecond. It is possible to increase the coincidence rate to the detriment of temporal precision by placing a filter reducing the spectral range in front of the photodetector. Such a filter is also necessary when the spectra of the two oscillators are not identical. Finally, in the case where the spectral overlap between the two oscillators is nil, the coincidences can be detected using a non-linear optical process (two-photon absorption, frequency sum etc.).

(36) The invention has been illustrated and described in detail in the drawings and in the previous description. The latter must be considered as illustrative and given by way of example, rather than as limiting the invention to this single description. Many variant embodiments are possible.

(37) In the claims, the word comprising does not exclude other elements and the indefinite article a/an does not exclude a plurality.